The long term objective is to understand the detailed mechanisms involved in transduction and light adaptation in vertebrate photoreceptors. The results may help our understanding of dysfunction of photoreceptors and other, related second messenger systems. It is known that the electrical response to light in vertebrate photoreceptors results from closure of light-sensitive Na channels. These channels are held open by cGMP in the dark, and close in light because a light-activated phosphodiesterase rapidly hydrolyses cGMP and so reduces its concentration. The phosphodiesterase is activated by rhodopsin via an intermediate G-protein, transducin. However, details of the biochemical steps and their control by ions such as Ca are poorly understood. Little is known about the corresponding biochemical processes in cones. One immediate aim is to determine whether the form of the light response in rods and cones can be accounted for by changes in the activities of the cyclase and the phosphodiesterase. The activities of the cyclase and phosphodiesterase will be determined indirectly from rapid solution substitution experiments. A second aim is to determine the origin of noise processes in rods and cones. A major goal is to investigate the biochemical pathway in single rods and cones by introducing chemicals or proteins into single photoreceptors using whole-cell patch electrodes. Similar techniques will be used to examine the relationship between proteins involved in phototransduction and those in other G- protein systems such as the hormone-activated adenylate cyclase system. The light-sensitive current will usually be measured using a suction electrode and external solutions will be changed using either rapid flow methods or internal perfusion of the suction electrode.